PURPOSE:To release the working accuracy and at the same time to facilitate an easy constitution of a series array with a fixed pitch of an optical axis, by setting the external size shorter than double as much as the pitch length of the optical axis for a poloprism of the pitch direction of the optical axis. CONSTITUTION:The outer diameter L of a poloprism 1 of the pitch direction of optical axes A and A' is set slightly smaller than double as much as the pitch length alpha of axes A and A'. Both axcs A and A' are positioned equidistantly from a face E1 where a minor prism part C1 reflecting the incident light of the prism 1 is adjacent with a minor prism part B1 radiating the light of the prism 1. A poloprism 1a having the same size and a relation of mirror image with the prism 1 is turned 180 deg. around the axis A', and a minor prism part B2 of the prism 1a has a contact with the lower part of a minor prism part C1 of the prism 1. Thus the concidence of direction is obtained between the axes A and A'. This facilitates an easy constitution of a series array having a fixed pitch alpha of the optical axis. Furthermore the common difference of production is released since the side face E1 has no contact with an adjacent face E2.
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